Thermoplastic pipe



Jan. 19, 1965 P. D. WOLFE THERMOPLASTIC PIPE 5 Sheets-Sheet 1 Filed Feb. 19, 1962 INVENTOR PAUL DILLON WOLFE mm IIIIIIIIII'III IIIHI ATTORNEY Jan. 19, 1965 P. D. WOLFE THERMOPLASTIC PIPE 5 Sheets-Sheet 2 Filed Feb. 19, 1962 FIG.2

INVENTOR PAUL DILLON WOLFE ATTORNEY Jan. 19, 1965 P. D. WOLFE 3,166,099

THERMOPLASTIC PIPE Filed Feb. 19, 1962 3 Sheets-Sheet 5 INVENTOR PAUL DILLON WOL FE BY W ATTORNEY sistance to impact.

United States Patent 3,166,099 THERMOPLASTIC PIPE Paul Dillon Wolfe, Wilmington, DeL, assignor to E. I. du Pont de Nernours and Company, Wilmington, Del., a corporation of Delaware Filed Feb. 19, 1962, Ser. No. 174,310 2 Claims. (Cl. 138-418) This invention relates to improved pipe made of acetel resin, and more particularly to pipe made of acetal resm having a critical degree of roll-orientation to produce maximum impact strength.

This application is a continuation-in-part of application Serial No. 27,316, now Patent No. 3,089,187, filed May 6, 1960.

' It has been known heretofore to manufacture thermoplastic pipe by extrusion. Such pipe has proved to be extremely valuable for many applications. Pipes made Patented Jan. 19, 1965 alkyl ethers or the like, using suitableinitiators known a to the art, with or without post-polymerization treatment to modify the character of the end groups or comonomer units.

Blends of polyoxymethylenes with minoramounts of other compatible, resinous materials, and particularly thermoplastic materials containing hydrogen bonding groups such as the polyamides, polyimides and the like from thermoplastics are. not subject to many of thetypes of corrosion which destroy metal pipes, and in this respect they resemble clay or ceramic pipes, but have the advantage over the latter of lightness ofweight and re- Resistance to impact also differentiates plastic pipes from metal structures. On impact, metal pipes tend to undergo ductile deformation, pinching and closing the pipe, Whereas plastic pipes rebound to substantially the original dimensions, or, in the event that the impact strength is exceeded, to fracture.

It has also been knownrthat themechanical properties of thermoplastics can be improved by molecular orientation. Molecular orientation'is produced by the mechanical deformation of thermoplastics, inparticular ing, greater degrees of molecular orientation and'great er improvements in properties are produced by greater deformations.

In the case of pipes made from acetal resin in which orientation substantially in the hoop direction has been achieved by rolling, a novel and surprising results has been found. Unlike properties such as the tensile strength which increase with increasing deformation, the impactstrength increases to'a maximum value at rela:

tively small amounts of deformation. Further deformation decreases the impact strength. This result is particularly surprising in view of the commonly accepted explanation of the mechanical phenomena associated with are included in the definition of acetal resin.

Stabilizers for thermal degradation, oxidative degradation or degradation by ultravioletradiation may be present. Minor amounts, offillers, coloringiagents, nucleating agents, plasticizers, or the'like may likewise" be present and are specifically included in the generic expression acetal resin, wherever employed in this specification and the appended'claims. 1

The process by which the pipe is produced and rolled may be understood 'by reference to the accompanying drawings. In the drawings:

FIGURE 1. is a side-elevation view, partly in cross section, of a rolling mill (with the mandrel omitted for the sake of clarity) which is-suitable for the practice partly crystalline thermoplastic resins. Generallyspeakof thtis" invention.

FIGURE 2 shows an end elevational ing mill of FIGURE 1. a 1

FIGURE 3 is a longitudinal cross sectional View showing. the construction of a water cooled mandrel suitable view of the rollfor use in the practice of this invention,

FIGURE 4 is an assembly view showing one embodiment of this invention .by means of which thermoplastic pipe may be extruded and rolled to produce extremely 7 long lengths of pipe in a unified operation;

Referringnow to the accompanying drawings:

FIGURES 1 and 2 are an elevation and end view of a mill suitable for" the practiceof this invention. In the drawings like. parts are numberedalike: The roll 1 of the inelastic deformation of thermoplastics, that. the prop erties are modified by molecular orientation, since molecular orientation evidently increases continuously with increasing deformation. a a

In the broadest'sense, therefore, this invention pro vides pipe of acetal resin rolled to produce a degree of extension in the circumferential direction of between about 1.05 and about 1.4.

, By degree of extension in the circumferential direction is meant the ratio of the mean diameter of the product pipe to the mean diameter of the unrolled pipe billet from which the product pipe is produced.

By acetal resin is meant a polyoxymethylene having a number average molecular weight of at least 15,000. There are several varieties of polyoxymethylenes which may be distinglished by thegroup which terminates the polymer chain substantially consisting of recurring (.CH O) units.- For example, there are the polyoxymethylene glycols in which. the terminating groups are hydroxyl, polyoxymethylene dicarboxyl-ates in which the terminating groups are esters such as acetate or propionate,'- and polyoxyrnethylene diethers inv which the are in the thermal stability and hyclr'olytic stability, poly- 3 and a screw 4. The shaft 2 rotates freely in roller bearings Sand which'support-the shaft and roll and are atframe is located a bevel gear 8 which is rigidly aflixed to p the shaft '2 with the key 9 and the locking nut' 10. f The gear and roll assembly mounted on the plate 7 is adjustably located with a' tongue and groove on the central plate 11 which is rigidly afiixed to the hollow driving shaft 12. At the other end the plate 7 is supported by the rod 13-,

1 locking screws being provided to rigidly atfix therod 13 to the plate 7. Two other roll assemblies eachidentical I with thatdescribed hereinabove are located at from.

Thefassemblies are supported by the common frame ll andby. bars 14, '15 and 16 which support the rod13and its equiva V alents, being s'ecured'tothese bars" by pairs of the taper. pins i7, 18, 19, 20,21 and 22. The bevel gear 8 (and its equivalents) rotate substantially *epicyclicly about. the fixed gear 23, which in turn is boltedjto the bearipg' frame I 24. The drivingcylinder 12 is supported byth e-bearing *j ZS'attached to'the frame 24, and by a second bearing and frame 26, attached rearwardly of the bearing 25.

Between the bearings 25 and 26 a thrust ri'ng27 'is' affixed Y to the hollow driving shaft 12, thereby supporting-the rolls against axial thrust. 25A .gear:.wheeli30 is also attached to the drive shaft 12 rearwardly of the ,SCLCOlld-jf 1:

each other about the principal axis of the mill.

amass i'assist the operation of the apparatus.

, structed to produce suchovertravel; 1

- 7 The-axes of the rolls are laterally. displaced so that they -junctionwiththeofiset. M H a jTurning now to FIGURE 3, there is shown a viewrin f section of a mandrel which may .be'employed in the prac-' bearing 6. Thisgea r wheel provides a means whereby the mill may be driven by a'variable speed electric motor and gear train (not shown in FIGURE 1). When the shaft 12 is rotated, the'rolls and gear assembly are driven epicyclicly about the central gear 23.

The rolls 1 (and its equivalents) have two conical surfaces 31 and 32. The bulk of the reduction .in-thickness iseiiected by the rolling action of the roll surface, 31 against the substantially conical surface of the man-. drel (not shown in these diagrams). Thesecond surface 32 provides aninitial grippingaction at; the cylindrical For the sake of si plicity in the followingi it' will be v assumed that the axis of the rolls, Spindles and gears intersectthe principal axis of-thei apparatus. In this case,

the action of the rolls maybe understood by connecting of the axis of the planetary rolls'with the principal axis of the entire machine.

the cones being, coincident, and the planetary. cones being defined by their respective. gears. If 'the surface of the rolls lies'in the surfaceof these rollingplanetary cones,

then the "action ispurely a rolling motion. However, if

thesurface of the rolls lies onfa cone having the same apex as the rolling planetary eone but having a greater Referring to FIGURE 4, an assembly view of a rolling mill coupled with an extruder is shown. The outlet end of an extruder 50, is shownfitted with a cross-head 51.

portioh .of the forming 'mandrel. and has been found to Thermoplastic material is introduced in the hopper 52 of the extruder and plasticated by the rotation of an extrusion screw and thence urged forward into the cross-head by the action of the screw. Within the cross-head, an annular passageway is defined by the outermost part of the die and by a water-cooled inner core 53. Cooling water is'supplied to the core by the tube 54. The exit water leaves by the annular space between the mandrel and the extrudate 55 and serves to cool the flared. central mandrel of the rolling mill. The thermoplastic is thus extruded as a tube, having a less diameter, andhaving a greater wall thickness than the desired end product.

This method for the -manufacture of tubing from thermoplastics produces stock having a greatly improved aconedefined by the line of contact of the centrally fixed I gear with an apex determined by the point ofintersection The planetary system is thus a sys tern of cones rolling on this central 'cone, the apices of all apicalangle,gtherr a shearing component of force at right a langlesto the line of contact is introduced which'tends to feed the thermoplastic under'the rolls. This. situation 7 may-be termed over-travel,gsignifying that the surface- 1 a of the cone is driven faster than the velocity required for true rolling contact. A small amount of over-travel is of decided advantage in the rolling of many thermoplastic tubes; The rolls shown in FlGURESj land 2 arei cona I do. not intersectonthe axis of the'mill'but pass by the principal; axis so that the least distance is from l/l0 to 1/1" of the diameter of thepipe stock from the principal surface over. stock produced by prior art methods, and

has been described in greater detail in a copending application Serial No. 27,315, now Patent No. 3,103,409. In addition to being more 'readily adapted to the process 7 of the. present inventionfthanprior art proce'sses,'the

improved inner surface i e-highly desirable for use-with the rollingtechnique,fsince a particularly good finish on the end product may. be obtained when such stock is used. By contrast, prior art processes for .the extrusion of pipe stock tend to produce tubing 'with'a rough inner surface.

containing flawswhich are magnified byfthe rolling process. H

After the stock leaves theextruder, it is gripped and urged forward by rolls 5 6v towards: the tube mill. The

' tubemill comprises the .water'cooled mandrel 57 which fis supported from the center of the cooled die cone by.

the rod 66.; The cooling water, somewhatheated from the internalfextr usion; core, flows through the annular passageway formed between the rod and the plastic extrudate 59, thence through and around the flare in the mandrel and out via the product tubing 58, thus servingto cool the mandrel.

axis. 1 This introduces a component of force which tends w to assist theintroduction ofthe plasticstock'intothe rolls. The magnitudeof the offset will not'in general-exceedthe radius of thepipestock,

Lubricantsmay be supplied to the outer surface ofgthe ipe'asit passes into themilLbut this is not an essential feature of the process.

. -The stock59 is urgedlintothe mill by the same gripping g devicejwhich serves to withdraw the tube from the ex:

A feature of the rolls, which is'of assistance to the feeding'action' of the offset, is thefpresence'ofa second conical surface whereby a'slightrolling' takes place prior f to the contact of the pipe stock with the flared; portion of} the mandrel. The. axial force. required-to pushthefstock through the mill is greatly reduced: by this feature in con-f tice of this invention in conjunction with the rolling mill shown in-FIGURES 1 and 2. i

The mandrel consistsof an outer their} havingfsub stantially the form of a truncated cone which merges into. a cylindrical section 46. The mandrel issupported by a thick-walledmetal tube -lto which it isattached. The

inner core is a conical plug' dzinto the" surface. of which is cut a series of deep circular. grooves, '43, 44,,fi5 and 46.. V Aslot'is cut in e achof the landsseparating these-grooves, V

adjacent lands being slotted at 180 to each otherso that a tortuous pathis provided'beneaththe surface of ,themam.

drel' shell. Inlet sf47 connectingthe first;.channel 4: .to

tinder. The rolls 6%, which "serve to'reduce the stock' 7 to the desired thickness, are driven by the epicyclie gear systen1 6l, "the rolls being driven by the rotation of the shaft 62, which,. in turn, is driven by the gear train excellent results, but. the included angle may be as little as 20.";or as great as 70 .f0r various embodiments ofthis the tube 41 extend through the-Qouter shell and outlets; 49

extend jthrough -the final land 48" to thelast groovegfi to.

providean e'xit forthe' cooling-watch; V I

,WhenQa mandrel is employed-in linerwith anextruder using a *cooled mandreljat thej-dieof the iextruder von which the therrnoplastic is extrudediinfthe-form of a tube, it {may be desirable tofpass the .wateninto the' mandrel" *through the annularspace found 'between'the supporting a rod and the inneiisurface-of the tube'g fi invention. Thejangleis governed inpart bytherequirement that the reduction in thickness'per pass of each roll is srriall sothat a substantially biaxial force system is v established. The. reduction in thickness" per revolution is givenby the expression:

and the ireduction per each roll is obtained by i i "t p s n by thei total number; or rolls.

Where At is the total reductionfin overall thickness of thei pi pe stockin inches-, is the feed' z ofthefsitock in inches; per minute, R-is the revolutions periminuteof the] p r rolling head and L is the axial length of the flare of the mandrel in inches. The rolls arejprefer'abl'y oli set qf t t h 'i 9?'intst s th ex b t e the impacting device.

It has been found that the best results are obtained where the least distance between rolls and mandrel decreases lineally with distance through the mill. Thus, where conical rolls are employed with a mandrel of 45 angle the mandrel having a maximum diameter of 2.195" and a minimumof 1.180", and the axis of the rolls were displaced /s" from the principal axis, it was found that the surface of the mandrel cone should be concave, with a radius of curvature of 6", by geometrical construction based on scale drawings. I V a The temperature of the mandrel must remain uniform in order to maintain a uniform product. However, it is preferred that the mandrel be heated to a temperature of about 60 C. for most thermoplastic materials. It has been found that a sharp reduction in the power requirements for rolling and an improvement in the quality of surface in the finished tubing takes place as the temperature of the mandrel is raised to about 60 C., the exact temperature varying somewhat with the plastic. Above a mandrel temperature of about 60 C., the power required to drive the mill remains substantially unchanged until the crystalline melting point is approached. It is essential that the mandrel temperature be kept well below the crystalline melting point of the thermoplastic material which is being fabricated in order to prevent melting of the material by the mechanical Work supplied. Generally speaking, the cooling fluid supplied to the mandrel should be kept at least 50 C. below the crystalline melting point of the polymer.

The rolling process may be modified in many respects, for example, the diameter of the stock may be increased by passing the stock through two or more mills in succession.

The impact strength is a relatively imprecise term, and the values obtained by test methods vary with the method of test. The results obtained by test methods are comparable for a given geometry of sample and provide an extremely valuable index of performance under actual operating conditions in the field.

In the following examples, the test procedure for im pact strength was as follows:

Samples of pipe are cut to a length of 5":%" and placed on an anvil consistingof a sheet of steel machined to provide a V surface with an angle of 170. A steel cylinder having a weight of 16.8 pounds and having a circular impacting face with a diameter 0.71 times the diameter of the pipe to be tested is used as By varying the diameter of the impacting weight with the diameter of the pipe in this manner, it has been found that the type of fracture pro-. duced remains substantially the same. Results for pipe of differing sizes may be compared by dividing by the weight per unit length. a

In order to perforrn the test, the pipe specimen is face at the bottom of' the cylinder and the top of the pipe specimen resting on the anvil, is adjusted to a value roughly equal to the height expected for failure of 50% of the samples. The cylinder is released andpermitted to drop onto the sample. After testing,- the sample is graded as a failure if it is cracked or broken in any way. i V

If the sample is not broken or cracked in any way, it is graded as support. If the test specimen is graded failure, the succeeding testis made; at a drop height lower by an interval I than the preceding test. If the test specimen is graded support, the next test is made at a drop height greater by I than the preceding test. The interval I is taken as A for a 50% break height of 2' or less and /2 for a 50% break height of more than 2'. Proceeding in this manner, a minimum of ten samples from the pipe are tested, and preferably thirty samples.

The results of these tests should be tabulated by height of drop, and the'number of failures. The mean break height is then determined by the following form a In this formula X is the mean break height, I is the interval, defined hereinabove, in feet, H is the lowest height of the drop weight, N is the number of failures at height H, and v (a is the number of intervals, I, between H and H The results of the test are conveniently expressed in foot-pounds by multiplying the mean break height X by the impact weight in pounds.

' angle of the rolling mill was 1720 measured from the/ The impact strength of pipes made from acetal resin depends somewhat on the degree of moisture present. In'order to eliminate this variable, the samples employed were conditioned to 50% relative humidity at23 C. for

five days preceding the test.

EXAMPLES The following examples show the properties of pipe axial line. Three rolls in the form of truncated cones were employed, the radius at the .apex of the cone being 0.925". was 28 with respect to the roll axes. The roll axes were set at an angle of 45 to the machine axis.

The properties of the rolled pipe so produced, together placed onthe anvil, which should be free from the with other pertinent fabrication data are shown in Table I.

Table I .Impact strength of roll-oriented acetal resin pipe Dimensions of Product Dimensions of Pipe Characteristics of Rolling Pipe (1n. Stock (111.) Extension in Gircum Impact Example No. ierential Strehgth,

\ Direction Billet 'Linear Ofiset ft.pounds Diameter Tlnckness Diameter Thickness Tcmp- Rate, of Rolls erature, in./min. (in.)

2. 400 0.070 2. 400 0.070 1. 0O (Extruded Control) 44 2. 390 0. 070 2. 150 0. I00 1. 14 45 15 1 9/16 250 2.380 0.075 1. 0.127 1. 6 V 45 '15 9/16 200 2. 380 0.075 1. 370 0.145 1 9 45 15 9/16 188 fragments of pipe remaining from previous tests and the weight is suspended vertically over the axis of the pipe from a stand having a reliable clamp. The height, measured as the distancebetween the impacting sur- In, other examples, the variation of impact strength at constant circumferential extension was studied. ",At axialq 1 Pipe stock was fabricated from this poly- The angle of the conical surface'of the mus;

' extensions between about beestablished, above extensions of about 1.25, a slight improvement in impact strength was detected which was, however, substantially less than the improvement demonstrated hereinabove with circumferential extension.

I claim: 7

1. As an article of manufacture, seamless pipe of acetal resin, roll-extended substantially, in the circumferential direction to-a degree of extension in the range between about 1.05 and about 1.4.

2.. The article of claim 1 which the acetal resin 1.0 and 1.25, no efiect could .containsbetween about 0.3 and 1.4%' by weight of carbon black.

3 References Cited in the file of this patent V UNITED STATES :PATENTS 2,244,208 Miles June 3, 1941 2,708,772 Moncrietf May 24, 1955 2,752,637 Walker et a1. July 3, 1956 2,761,336 Greene et a1. Sept. 4, 1956 2,961,711 Diedrich et al. Nov. 29, 1960 3,035,302 1962 Lysobey May 22, 

1. AS AN ARTICLE OF MANUFACTURE, SEAMLESS PIPE OF ACETAL RESIN, ROLL-EXTENDED SUBSTANTIALLY IN THE CIRCUMFERENTIAL DIRECTION TO A DEGREE OF EXTENSION IN THE RANGE BETWEEN ABOUT 1.05 AND ABOUT 1.4. 